A typical gas turbine engine includes a compressor section, a combustor and a turbine section. Working fluid flowing through the gas turbine engine is compressed in the compressor section to add energy to the working fluid. Most of the compressed working fluid exits the compressor section and enters the combustor. In the combustor, the working fluid is mixed with a supply of fuel and ignited. The products of combustion are then flowed through the turbine where energy i s extracted from the working fluid. A portion of the extracted energy is transferred to the compressor section to compress incoming working fluid and the remainder may be used for other functions, such as thrust or shaft horsepower.
Gas turbine engines are required to function efficiently over a range of operating conditions. For a gas turbine engine used in aircraft applications and having a single stage combustor, low power operation corresponds to idle and high power operation corresponds to take-off, climb and cruise. At low power, fuel/air ratios are kept relatively low but above blow-out levels. Blow-out occurs when the fuel/air ratio within the combustor drops below a lean stability limit.
The combustion process generates numerous byproducts such as smoke particulate, unburned hydrocarbons, carbon monoxide, and oxide of nitrogen. Production of oxides of nitrogen increases as the operating temperature and residence time increase. Reducing the operating temperature may reduce the power output of the gas turbine engine. Reducing the residence time, defined as the amount of time the combustion mixture remains above a specific temperature, may result in less efficient combustion and higher production of carbon monoxide.
For environmental reasons, these byproducts are undesirable. In recent years, much of the research and development related to gas turbine engine combustion has focused on reducing the emission of such byproducts.
A significant development in gas turbine engine combustors has been the introduction of multiple stage combustors. A multiple stage combustor typically includes a pilot stage, a main stage, and possibly one or more intermediate stages. An example of such a combustor is disclosed in U.S. Pat. No. 4,265,615, issued to Lohmann et al and entitled "Fuel Injection System for Low Emission Burners".
At low power only the pilot stage is operated and the combustor is equivalent to a conventional single stage combustor. At high power the pilot stage and one or more of the other stages is operated. Having multiple stages reduces the residence time within each particular stage relative to having a single large combustion chamber. The lower residence time results in lower production of oxides of nitrogen. As a result of having multiple stages rather than a single stage, the emission of unwanted combustion byproducts is reduced.
A fuel control system for a multi-stage combustor must be responsive to the operator's demands and while maintaining efficient operation and ensuring the gas turbine engine is operated in a safe manner. Responsiveness of the combustor requires the fuel control to be able to supply the thrust demanded by the operator without undue delay. For multi-stage combustors, this also means that transitions between pilot only operation and staged operation should be smooth and prompt. Safety concerns are the avoidance of blow-outs and stalls. Blow-outs occur when the fuel to air ratio F/A within the combustor falls below the level needed to maintain combustion. Stall may occur if the combustor generates excessive back pressure on the compressor.
An example of a fuel control system for a multistage combustor is shown in U.S. Pat. No. 4,903,478, entitled "Dual Manifold Fuel System" and issued to Seto et al. This patent discloses a fuel system having two manifolds, one for each stage, and a shutoff valve between the fuel control and one of the manifolds. The shut-off valve opens and closes in response to a signal from a digital electronic engine computer. If the shut-off valve is open, fuel flows to both manifolds; if the shut-off valve is closed, fuel flows to only one manifold.
Another example is shown in U.S. Pat. No. 4,726,719, entitled "Method of and Apparatus for Controlling Fuel of Gas Turbine" and issued to Takahashi et al. This patent discloses a fuel control system in which the fuel control valve is controlled by a load control signal during normal operation and by a fuel flow rate signal during switches between single stage operation and two-stage operation. This patent also discloses using a predetermined time period during which the fuel control valve is controlled by the fuel flow rate signal.
The above art notwithstanding, scientists and engineers under the direction of Applicants' Assignee are working to develop effective and responsive fuel control systems for multi-stage combustors.